Going by thicker = warmer, the Browns have a thicker A, .224" vs .216", but for the rest the Martin M620's are thicker. .260, .291, & .244" for the browns and .0280, .0340, .0256" for the M620's. Any idea which set would sound warmer overall, or should I just try them both?
90% of what you are going to hear as 'brighter' tone is going to be from the A string more than anything else, and the primary factor at play is because the thinner string will sustain longer than most of the other strings due to having less tension than a comparatively thicker A string...
THUS, the Worths 0.0224" A string will stand out 'less' than the Martin M620 0.0216" A string in terms of hearing perception...
Also, the Worth browns, whichever gauge set, tend to be softer, or mellower, seemingly on account of whatever dye is used in the composition of the fluorocarbon to make them actually brown. I am not privy to any string formula, nor am I a chemist or expert in textiles, but it is a guess, and a shared conclusion with the rest of the UU Hive Mind that believes Worth browns to be softer/mellower than Worth Clears....
Also Fremont makes both clear and black fluoro strings. The 'Blacklines' are also popular on Fluke and Flea ukes, but in my experience with 8 different sets, purchased at different times from 3 different vendors, at least one string from each set has frayed and broken at some point, all of which had less than 40 hrs of playtime. Other folks have not mentioned this happening to them, but I am hesitant to want to use any Fremont Blacklines strings on anything other than a soprano, on account of it having less tension than concert or tenor, and therefore/hopefully less likely to break, since they are not cheap at a starting price of $8/set and up most places that sell them.